The use of pulse oximeters to noninvasively measure a patient's heart rate and blood oxygen saturation is well known. In general terms, noninvasive measurement of blood oxygen saturation by a pulse oximeter typically requires the transcutaneous illumination of a portion of the patient's blood-perfused tissue by light at two or more wavelengths. Changes in the amount of blood in the tissue during a blood pressure pulse change the amount and character of the light detected by the sensor's photodetector. The amounts of light transmitted through the tissue at each wavelength may be compared to calculate to what degree the blood flowing through the tissue is saturated with oxygen. A more detailed discussion of the principles of pulse oximetry may be found in U.S. Pat. No. 4,653,498.
Pulse oximetry sensors fall into two general categories. Transmissive pulse oximetry sensors shine light through opposed blood perfused tissue surfaces, such as a finger or an ear, by disposing the light emitters and photodetectors on opposite sides of the tissue. Examples of such sensors are presented in U.S. Pat. Nos. 4,685,464 and 4,830,014. Transflectance sensors, on the other hand, emit light into and detect light from the same side of the tissue. An example of a transflectance sensor is the Nellcor Incorporated model RS-10 sensor.
With any pulse oximetry sensor, the quality of the measurement depends in part on the concentration of blood (relative to other tissue structures) in the portion of tissue illuminated by the sensor and in part on the magnitude of the pulsatile changes in the amount of blood in the tissue. Fingers are a preferred sensor site for transmissive sensors because of the fingers' relatively large number and concentration of blood vessels. However, well-perfused sites such as fingers are not always available. In addition, where it is desired to monitor the patient for an extended period of time, a sensor which is attached to a finger could be very awkward and inconvenient for the patient and could interfere with and restrict the patient's movements. Furthermore, movement of the sensor when the patient moves his or her hand could cause errors in the readings of the sensor. These situations could dictate the use of a transflectance sensor placed on the patient's torso, head, or some other part of the body that is convenient and accessible.
The torso, however, has a lower concentration of blood vessels near the skin surface than fingers have. In addition, blood flow to sensor sites on the torso or on any other part of the body may be restricted due to the effects of ambient temperature, systemically-acting vasoconstricting drugs in the patient's blood stream, or low patient blood pressure. The prior art has attempted to address this low perfusion problem in several ways.
Some prior art pulse oximeter sensors have used heaters to dilate the blood vessels at the sensor site, thereby increasing blood perfusion. See, e.g., U.S. Pat. Nos. 4,926,867 and 5,007,423. Heaters have also been used with other transcutaneous blood characteristic measuring devices, as shown in U.S. Pat. Nos. 3,628,525; 4,488,557; 4,324,256; 4,517,982; 4,534,356; 4,536,274; and Re. 31,440. The use of heaters raises the cost and complexity of the sensors, however, and presents the possibility of tissue burns.
An early oximeter sensor called the "Cyclops" is discussed in W. G. Zijlstra and G. A. Mook, Medical Reflection Photometry, pp. 50-77 (Royal VanGorcum Ltd., Assen, 1962). Unlike a pulse oximeter sensor (which distinguishes the arterial blood optical signal from the optical effects of venous blood and tissue by using only the AC component of the optical signal), the Cyclops sensor compared the optical signal from exsanguinated (i.e., bloodless) tissue with the optical signal from the tissue in a normal, blood-perfused state. Because the parameter of interest is the oxygen saturation of arterial blood, the Cyclops increased the ratio of arterial blood to venous blood in the tissue by "arterializing" the area through the iontophoretic application of histamine phosphate, a direct vasodilator and counterirritant. A voltage is applied to the patient's skin to drive the histamine phosphate into the tissue by a process called histamine iontophoresis.
Other transcutaneous blood characteristic measurement devices have used vasodilators to increase blood volume in the measuring region (U.S. Pat. Nos. 4,296,752, 4,488,557, and Re. 31,440). While these sensor systems do not require the use of iontophoresis, they do require the extra step of manually applying the chemical to the skin surface topically before a sensor is placed over the application area. In addition, none of these sensors is an optical sensor. The thick and irregular layer of chemicals these sensors use could adversely affect the optical performance of a pulse oximeter sensor through shunting, diffusion, reflectance or color shifting of the transmitted and received light.
Transdermal devices for the delivery of a drug to the skin have been known for some time, and representative systems which utilize rate-controlling membranes and in-line adhesives are disclosed in U.S. Pat. Nos. 3,598,122, 3,598,123, 3,742,951, 4,031,894, 4,060,084, 4,144,317, 4,201,211, 4,379,454 and 4,908,027, which are incorporated herein by reference.
A transdermal device for delivering certain vasodilators such as nitroglycerin is disclosed in U.S. Pat. Nos. 4,661,105, 4,725,272 and 4,849,226. These vasodilators are used to relieve the pain associated with angina pectoris, for the prevention of angina, in hypertension, for relaxation of involuntary muscles of blood vessels, for increasing the flow of blood therein and for increasing oxygenation from vasodilation, mainly for increasing the supply of oxygen to the heart. The transdermal device is designed to provide a higher dosage of the vasodilator through the skin to give the desired therapeutic result.